Internal combustion engines use only a portion (for example, approximately 31% to 38% in some cases) of the supplied fuel energy due to heat wasted, friction, incomplete combustion, and others. In addition, approximately 3-17% of the supplied fuel energy can be used to maintain the engine operation during standby and another 1-2% can be used to operate accessories. Therefore, it can be advantageous to utilize the waste energy, typically in the form of thermal and chemical energy, to improve the overall vehicle system fuel efficiency.
One approach to utilize the waste energy is to arrange a solid oxide fuel cell (SOFC) in an exhaust system of an internal combustion engine. Specifically, the U.S. patent application Ser. No. 2004/0177607 describes an internal combustion engine with a SOFC in an exhaust system. The SOFC is of a structure that fuel can be reformed inside the fuel cell. In the '607 reference, one embodiment includes a fuel adding injector disposed between the engine and the SOFC (see FIG. 8). The fuel added to the exhaust passage can be used as power generation fuel for the SOFC. The embodiment also includes a heat exchanger and an air pump. The air with its temperature raised in the heat exchanger is introduced into the exhaust passage at the upstream side of SOFC so as to raise the wall surface temperature of the exhaust passage and the exhaust gas. Thus, evaporation of the fuel added from the fuel adding injector can be advanced.
However, the inventors herein have recognized several disadvantages of such an approach. For example, additional devices such fuel pump, fuel adding injector, air pump, and heat exchanger are required for the operation of the SOFC. These separate air and fuel supply systems plus the heat exchanger can be expensive or may become degraded over time. Further, the system may only use fuel cells that can perform fuel reformulation. Furthermore, the reformation of fuel in the SOFC may cause emissions such as NOx emissions.